Alrighty...
First off +1 on everyone saying to upgrade from the D90. To be brutally honest, that was a great camera back in the day but anything on the market would be a full torso above it in terms of performance. How old is the Sigma lens? That'll be a big deciding factor. IF the glass is fairly recent and works well, I'd keep it and upgrade later if needed. Hell, I'm using a 10 year old (ish) 70-200mm f/2.8G on my Nikon D850 without issue or resolution problems. Good glass lasts a long time.
Personally I'd go with the D500. That'll last her a good long time. Add in the memory card and reader and you're at $2000. IMO I'd toss in a good bag to go with it, top off her cleaning supplies with new stuff (add in a new Visible Dust hepa rocket blower, sensor loupe, etc) maybe replace the extra Watson battery that comes with the D500 with an OEM one, grab a focus calibration target, etc. Basically set it up to get the best performance out of the camera possible.
Barring the D500, the D7200 is a damned good DX camera. A lot of us used them for years flawlessly. I only sold mine to move up to the D850. The D7200 and D750 are on par with each other largely, one is a DX and the other is FX but are pretty comparable really.
BUT the D500 blows both out of the water in terms of ISO performance, frame rate, etc.
For $2500, this is what I'd recommend:
Nikon D7200 refurbished $770
Nikon D500 $1800 with free Nikon battery grip (normally $350 on it's own)
Nikon D750 $1400 with free Nikon battery grip (normally $350 on it's own)
D500 specific:
120gb Sony G series XQD card $222
Sony XQD reader $70